Method of silver coating organic surfaces



Patented Mar. 8,1949

METHOD OF SILVER COATING ORGANIC SURFACES Lawrence E. Martinson, Rochester, N. Y., and Kuan-Han Sun, East Pittsburgh, Pa., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application September 7, 1946, Serial No. 695,552

This invention relates to depositing metals on organic plastic surfaces and more particularly to a method of preparing such surfaces so that they will receive an adherent metal coating.

For many years, glass surfaces have been silvered by the chemical deposition of metallic silver from a. bath containing a silver salt and a reducing agent. A strongly adherent silver layer may be obtained if the glass surface is first treated with a solution of stannous chloride before placing it in the silvering solution.

When this method was applied to the plating of organic plastics, it was found that the plating did not adhere well to the non-polar surfaces of the plastics.

One method to improve the plating adhesion on cellulose derivative plastics is to treat the surface before metallizing first with one or more solutions capable of allowing a superficial soaking of the cellulose derivative but without rendering it capable of reducing the metallizing solutions applied to the surface, and then with a second bath containing stannous chloride and preferably phenol.

Another method for preparing the surface of organic cellulose ester plastics for adherent metal plating is to pretreat the surface of the plastic in a single bath containing a stannous compound dissolved in a mixture of ethyl alcohol and water. The concentration of alcohol is such that it assists penetration of the stannous reducing agent into the surface of the material but does not dissolve or destroy the material of the surface.

Still another method of overcoming the reluctance of a cellulose ester surface to take a silver plate is to process the ester surface in a saturated solution of potassium dichromate in sulfuric acid and then treat the surface with stannous chloride before depositing the metallic silver coating from a. silvering solution on the surface. The dichromate solution attacks the surface of the cellulose ester and apparently permits the stannous chloride to penetrate the surface and, therefore, subsequentl bind the silver coat to the surface.

Another method includes sand blasting the surface of the plastic article to depolish it and then coating the surface in a stannous chloride bath followed by a treatment in a silvering solution.

In all of these processes, the surface of the plastic article is in some way changed physically to improve the adhesion of the metal layers thereon.

In many cases, such surface changes in the plastic article are undesirable since they perceptibly change the dimensions and clarity of the article.

4 Claims. (Cl. 117-35) An object, therefore, of the present invention is an improved method for preparing the nonpolar surfaces of plastic articles, for coating with an adherent metallic coating, by which substan-- tially no deformation of the surfaces occurs.

Another object of the invention is to provide a method for applying intially a thin adherent metallic layer to a plastic surface upon which may be electroplated various metals.

Another object of the invention is to provide a method for preparing the surfaces of natural organic materialsfor silvering from silvering solutions.

Still another object of the invention is an improved method for silvering the surfaces of organic plastics which is comparatively simple and has fewer operative steps than the heretofore known processes.

It is a well known fact that silver adheres readily to glass when deposited from a silvering solution because the surface of glass is polar in nature. We have discovered that such metals when similarly deposited do not adhere readily to organic plastics because plastics have non-polar surfaces. For the same reason, metals do not deposit on threads, filaments and fabrics of synthetic materials of an organic or resin nature. A feature of our invention is, therefore, the treatment of various plastic and non-polar surfaces to produce a polar surface thereon which will attract and hold metal atoms.

In accordance with the invention, these and other objects are attained by applying to the surface of the plastic, or other surface of organic origin, compounds containing organic and inorganic groups which have an organic chemical affinity through organic groups for the plastic surface, and which also form on hydrolysis inorganic polar groups which have an inorganic chemical affinity for metal atoms. This layer is then silvered by any of the usual silvering processes. Various metals including silver may be electroplated, if desired, on the thin silver layer, and the plated coating will not tear off the plastic base during subsequent usage.

The compounds which we have found advantageous to employ for this purpose are organo metal halogenides such as are represented by'the general formula RnMXi n in which R represents an organic radical such as CH3, C2H5, CsHs, etc), 11. has a value of 1 to 3, M is a metal selected from the class consisting of Si, Ti, Sn, Ge, and X represents a halogen atom. These compounds all have the characteristics of forming a close range attraction through their organic radicals with 3 the surface of the plastic or organic materials being coated and have free halogen atoms which are easily hydrolyzed and permit a polar attraction or even a. direct chemical binding for any metal atoms placed thereover.- All these compounds are anhydrous liquids with high vapor pressures. They hydrolyze easily and fume in ordinary atmospheres.

Because these compounds employed in our invention all have high vapor pressures, they may be applied to the surface of the plastic article by exposing the plastic article to the fumes. After the article is exposed to the fumes, a thin layer, perhaps of the order of a few hundred molecules thick is firmly attached to the plastic surface. This layer is then hydrolyzed in air to A cellulose propionate sheet was given exactly the same treatment as described in Example I except dlmethyl titanium dichloride was employed in place of the silicon compound, and a smooth form MOH or MO-M linkages with O- or OH- groups available at the surface to attract metals placed thereover. This hydrolyzed layer is then immediately brought into contact in any suitable manner with the usual type of silvering solution from which metallic silver deposits on the hydrolyzed layer. This process dispenses with much equipment normally required when, as in the prior art processes, the article was roughened, immersed or drawn through a solution of stannous chloride, washed ofi, dried and so forth before being metal coated.

The formation of this intermediate layer for coating may be schematically shown as follows, R and X having the values given above.

Silver Layer 0 o Organic Body The invention is further illustrated in the following examples:

/\ RR R.

R RR

EXAMPLE I A sheet of cellulose acetate was drawn slowly across a bath of fuming dlmethyl silicon dichloride, (CH3)2SiCi2, and permitted to hydro- 13-1..v for a few minutes at the atmosphere of the room. The sheet was then drawn through a. freshly mixed silvering solution containing the following components in the amounts specified.

Enough NH40H is added to solution B to redissolve any brown precipitate which may be formed. Solution 0 is then added to solution B until a slight darkening is produced. A fresh mixture of 1 part of A is added to 4 parts of the combined B and C to give the silvering solution.

A very adherent silver surface was thus placed on one side of the acetate sheet. The thickness of the silver layer thus, formed will depend somewhat on the time the acetate sheet remains in contact with the silvering solution. If both sides of the sheet are to be silvered, then both sides may be exposed to the fumes of the polar comadherent silver layer was coated thereon.

EXAMPLE III A cellulose acetate butyrate sheet was treated as described in Example I except that dlmethyl tin dichloride was employed instead of the silicon compound, and an adherent silver layer was affixed thereto.

' EXAMPLE IV A molded article made of methyl methacrylate was treated by the process as set forth in Ex-' ample I but employing dlmethyl tin dichloride as the bonding material, and a smooth adherent silver layer was coated thereon.

EXAMPLE V A plastic sheet of cellulose acetate butyrate was coated with a thin layer of dimethyl germanium dichloride and an adherent silver layer was deposited thereon in accordance with the process of this invention.

Other plastic materials such as polystyrene; vinyl resins such as vinyl chloride, vinyl acetate, urea, phenol or melamine formaldehyde resin may also be coated in accordance with the invention, it being generally applicable to all such plastic materials. I

The process is also applicable to prepare surfaces of threads-and filaments of plastic origin for silvering by standard methods, including filaments and threads of cellulose acetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose acetate butyrate, polymeric amides, regenerated cellulose, viscose and so forth. Also fabrics made from any of these materials, or mixtures thereof, may be metallized in accordance with our invention.

While the process is particularly suitable for preparing the surface of synthetic plastics to receive a metal coating, the process is also applicable to prepare the surfaces of natural materials such as threads, filaments or fabrics made from wool, silk, cotton, flax, linen, ramie, jute, and may be employed in silvering glass, silicone fibers, and other materials of a ceramic nature.

We claim:

1. The method of silver coating an organic plastic surface which comprises applying to the surface an organo metal halogenide represented by the general formula RnMXt-n in which R is a radical selected from the class consisting of methyl, ethyl and phenyl radicals; M is a metal selected from the class consisting of Si, Ti, Sn, Ge; X is a halogen atom; and n has a value of 1 to 3, and further treating the surface with a reducible silver solution and a reducing agent for the silver solution whereby metallic silver is precipitated onto the plastic surface in a ten aciously adhering layer.

2. The method of silver coating an organic plastic surface which comprises applying to the surface dlmethyl silicon dichloride, and further treating the surface with a reducible silver solution and a reducing agent for the silver solution whereby metallic silver is precipitated onto the plastic surface in a tenaciously adhering layer.

3. The method of silver coating an organic plastic surface which comprises applying to the surface dimethyl titanium dichloride, and further treating the surface with a reducible silver solutlon and a reducing agent for the silver solution whereby metallic silver is precipitated onto the plastic surface in a tenaciously adhering layer. 4. The method of silver coating an organic plastic surface which comprises applying to the surface dlmethyl tin dichloride, and further treating the surface with a reducible silver solution and a reducing agent for the silver solution whereby metallic silver is precipitated onto the plastic surface in a tenaciously adhering layer. LAWRENCE E. MARTINSON. KUAN-HAN SUN.

6 REFERENCES crran The following references are of record in the file of this patent:

UNITED s'r 'ms PATENTS 

